The long-term objective is to determine the mechanisms of chemical carcingensis in the breast tissue. Our current focus is to study the role of mammary cytochrome P450 (P450) enzymes in chemical carcinogen- induced DNA adduct formation in the mammary gland (MG) using tissue-selective gene knockout mouse models. It is well-documented that the microsomal cytochrome P450 (P450) enzymes play a critical role in the metabolic activation of chemical procarcinogens. However, it has been difficult to determine whether, following systemic exposure to a given procarcinogen, the bulk of the activated carcinogen, or its DNA adducts, in the breast tissue is derived from local metabolism or from the liver;the latter organ is generally much more active than extrahepatic target tissues in the metabolic activation of procarcinogens. The problem becomes even more complicated for procarcinogens, such as 7,12-dimethylbenz[a]anthracene (DMBA), that require multiple steps of metabolic activation, given the potential contributions of hepatic or breast tissues to the formation of not only the ultimate carcinogen, but also the intermediate metabolites, such as DMBA-trans-3,4-dihydrodiol (3,4-diol-DMBA), that are precursors to the ultimate carcinogen. Our hypothesis is that target-tissue P450- mediated metabolic activation plays an important role in DMBA-induced formation of DNA adducts in the mammary gland (MG);however, the extent of this target-tissue contribution will vary according to the developmental stages and physiological states of the MG. To test this hypothesis, we will modulate P450 activities in the liver and extrahepatic tissues (including MG) through conditional deletion (or suppression) of the gene for P450 reductase (Cpr), an enzyme required for the activity of all microsomal P450 enzymes. In this two-year pilot project study, we will determine the relative contributions of hepatic and extrahepatic (including the MG) P450 enzymes in the metabolic activation of DMBA, a mammary procarcinogen, and its intermediate metabolite 3,4-diol-DMBA, in newly developed mouse models with tissue-specific suppression of P450 activities, either in the liver alone (designated as liver-Cpr-null mouse), or in all tissues except the liver (designated as extrahepatic-Cpr-low mouse) (Aim 1). Furthermore, we will develop and characterize MG selective Cpr null models (designated as MG-Cpr-null mice), for direct determination of the role of MG P450 enzymes in the MG DNA-adduct formation induced by DMBA and 3,4-diol-DMBA (Aim 2). The knowledge gained from studies will improve the design of epidemiological studies that attempt to associate environmental chemical exposure, and/or genetic polymorphisms in P450 enzymes, with the incidence of breast cancer in humans. It may also help with designing mechanism-based approaches to the prevention of breast cancer. PUBLIC HEALTH RELEVANCE: This research will further our understanding of the mechanisms of chemical carcinogenesis in the mammary gland induced by environmental chemicals, and provide novel insights for mechanism-based approaches to the prevention of breast cancer.